How do I keep the aspect ratio of an image imported to the 3d viewport as a plane?
Think of it this way: 

A scene in blender is created in a virtual world.

The units for that world and all of the objects are blender units (that coincide with meters. one blender unit is one meter, the units can also be set to imperial measurements or KM or mm if you wish)

When you import an image as a plane, it will create a plane that is one blender unit tall (that can be changed in the options as you import the image), and that has the aspect ratio of the imported image. 



The number of pixels in the image does not determine the size of the plane.

If you need to create planes that are different sizes and have their size relative to their dimensions in pixels, you could do that manually using the information on this link:

How to map an Image on a plane manually?

Do not touch the aspect ratio on the dimensions for the render unless you know what you are doing and need to export an image that is using anamporphic pixels. Read: Rendered aspect ratio different from the viewport&#39;s camera view